In electrophotography, color image formation is based on a three-color process such as a subtractive color process and generally consists of forming at least three electrostatic latent images, developing the latent images with at least three different color toners, and reproducing the original color on copying paper. Requirements for the color toners used here are severer than in black-and-white development. That is, the toners are demanded to have mechanical and electrical stability against such external factors as shocks and humidity, proper color developability, and color preservability.
Conventional processes for producing full color toners mostly comprise melt-kneading a colorant and a binder resin, grinding, and classifying to obtain particles of a prescribed size. Where a pigment is used as a colorant, since it undergoes agglomeration on drying, re-grinding or any other means for size reduction is required. To improve this, it has been proposed to prepare a toner by a process comprising incorporating an aqueous dispersion of finely divided pigment particles obtained by an acid paste process or an acid slurry process into an organic solvent solution of a binder resin, heat treating the mixture, and mixing the resulting pigment dispersed-resin with a binder resin as disclosed in JP-A-62-127847 (the term "JP-A" as used herein means an "unexamined published Japanese patent application").
However, the proposed process turned out to result in insufficient brightness and clearness of an image. This disadvantage is particularly conspicuous when a toner image is formed on an over-head-projector sheet (OHP sheet) to obtain a projected image. In order to obtain a distinct color image, particularly a Victorian full color image, by overlapping several color toner images, a magenta toner, a yellow toner, and-a cyan toner are overlaid one after another and fused together to obtain a second-order color and then a third-order color. For example, where a second-order color is formed by overlaying two color toners, a color difference between a theoretical second-order color and the second-order color actually obtained is decided by transparency of the toner layers. When at least the upper toner layer has satisfactory transparency, the light reflected on the lower ink layer(s) is close to that of the second-order color assigned to the characteristics of the pigments themselves to achieve satisfactory color reproduction. However, sufficient transparency could not be obtained with conventional toner materials, failing to obtain a satisfactory overlaid image.
On the other hand, in the production of full color toners, various attempts have been made to obtain excellent graininess for a half tone of a digital image, and use of toner particles having a diameter of not more than 8 .mu.m has been proposed. In this case, however, the effect of improving graininess is small with high pigment concentrations.